The present invention relates to a lens unit for a multibeam scanning device, which simultaneously scans a plurality of light beams emitted by multiple light emitting elements on a scan target surface (e.g. the surface of a photoconductive drum) by dynamically deflecting the light beams by use of a deflecting system.
Scanning devices for scanning a light beam emitted by a light emitting element on a scan target surface by dynamically deflecting the light beam by a deflecting system have been widely known. However, image formation speed of such single-beam scanning devices (forming images by scanning only one light beam on the scan target surface) is generally low. For increasing the image formation speed, multibeam scanning devices, which simultaneously scan a plurality of light beams emitted by multiple light emitting elements on the scan target surface by dynamically deflecting the light beams by use of a deflecting system, have recently been proposed (e.g., Japanese Patent Provisional Publication P2001-194605A) and have been in practical use widely.
A lens holding mechanism capable of preventing distortion of a lens by absorbing deformation of the lens caused by temperature variation has been proposed in Japanese Patent Provisional Publication No. HEI 07-191247 (pages 2–5, FIGS. 1, 6 and 9). The lens holding mechanism places elastic material between the lens and a holding frame which holds the lens so that temperature-dependent variations (especially, lens deformation caused by thermal expansion at high temperatures) will be absorbed by the elastic material. The elastic material prevents the lens distortion by absorbing the lens deformation mainly in the radial direction.
The patent document Japanese Patent Provisional Publication No. HEI 07-191247 also proposes a countermeasure against lens deformation in the thrust direction, in which lens distortion caused by the lens deformation in the thrust direction is avoided by fixing the lens by vertically sandwiching the lens between the holding frame and a lens retainer, by bonding the elastic material to the lens and vertically sandwiching only the elastic material between the holding frame and the lens retainer, etc.
In these multibeam scanning devices, it is essential to maintain each distance between beam spots (formed on the scan target surface by the light beams) with high precision corresponding to a prescribed desired resolution throughout the scanning period. In other words, relative positions among optical paths of the light beams have to be maintained substantially constant. However, when an expensive one-chip (integrated) multibeam laser diode unit or a multi-chip laser diode unit is employed for the multibeam scanning device, it is impossible in many cases to maintain the distances among the beam spots formed on the scan target surface (measured in the scanning direction) since each component of the chip moves slightly due to temperature variation.
Further, the position of the collimating lens employed in the multibeam scanning device, as the outlet of the laser diode unit for the light beam, has an important effect on the position of the beam spot on the scan target surface. For example, if the collimating lens slightly moves relative to the light beam emitted from the laser diode, the beam spot on the scan target surface moves three to ten times as long as the displacement of the collimating lens. Therefore, in conventional multibeam scanning devices, a sensor is generally placed at a position equivalent to the scan target surface and each distance between the beam spots is monitored and fed back, that is, each distance between the beam spots is maintained to be constant by means of a closed-loop system.
However, providing the multibeam scanning device with such a feedback control mechanism (closed-loop control mechanism) leads to upsizing, complication and high cost of the device.
In the method proposed in Japanese Patent Provisional Publication No. HEI 07-191247, the lens are vertically sandwiched between the holding frame and the lens retainer in order to resolve the lens deformation problem in the thrust direction, each component of the lens holding mechanism contracts in low ambient temperatures, by which clearance occurs among the components and the lens moves relative to the holding frame. Therefore, in the highly sensitive multibeam scanning devices, the lens holding mechanism having such composition can not successfully maintain the beam spot intervals on the scan target surface (measured in the scan direction) to be constant.
Similarly, in the method of Japanese Patent Provisional Publication No. HEI 07-191247 bonding the elastic material to the lens and vertically sandwiching only the elastic material between the holding frame and the lens retainer as the countermeasure against lens deformation in the thrust direction, injecting adhesives between the small-sized lens installed in the multibeam scanning device and the elastic material is difficult, and the increase of steps in the manufacturing process leads to high cost. Further, once the elastic material is bonded to the lens, the lens cannot be separated from the elastic material and thus both have to be discarded when quality of one of them deteriorates. Further, in cases where such adhesives are used, thermal expansion of the adhesives accompanying temperature variation might have ill effects on the lens.